This grant proposal is a competitive renewal of a project we have pursued for 10 years directed at understanding the mechanism of action of the vitamin D receptor (VDR) as the mediator of 1,25-dihydroxyvitamin D (1,25D) action. The project has supported the publication of many original research papers as well as reviews of the field, editorials, chapters and the editing of two multi-authored books. The next support period will continue our investigation into the role of the VDR in clinically relevant situations in which 1,25D activity is enhanced or suppressed by either genetic factors (mutations or polymorphisms) or physiologic factors (regulators of VDR abundance or action). The clinical diseases most relevant to the project are: Aim I, rickets; Aim II, osteoporosis; and Aim III, prostate cancer. Specific Aim I will investigate Hereditary Vitamin D Resistant Rickets (HVDRR) and elucidate loss of function mutations causing rickets and a gain of function mutation causing hypercalcemia. An additional project will study the use of 1,25D analogs to activate mutant VDRs and treat HVDRR patients. Specific Aim II will focus on the structure of the VDR and will examine factors that regulate the abundance of receptors and the capacity of the VDR to mediate 1,25D actions. Sub-Aim 1 will investigate the VDR promoter. Sub-Aim 2 will investigate the VDR promoter variants leading to N-terminal isoforms and evaluate their differential functional potency and tissue distribution. Sub-Aim 3 will investigate VDR polymorphisms and the ability of the variant receptors to respond to 1,25D and analogs with target gene activation. Specific Aim III will focus on 1,25D actions. The sub-Aims will involve cDNA microarray analysis to identify new target genes in prostate cancer and bone; a further analysis of IGFBP-3 as a target gene that mediates some of the anti-prostate cancer activity of 1,25D; and an analysis of the negative vitamin D response element (nVDRE) in new target genes including the estrogen receptor and IGFBP-3. Many of the projects will be carried out in collaboration with leading researchers in their respective fields. Comprehensive preliminary data and an extensive track record support our ability to successfully conduct this large, multi-phasic study. The long-term goals of the grant proposal are to provide further insight into the mechanism of 1,25D action and to understand the role of the VDR in affecting disease risk, pathogenesis and treatment of rickets, osteoporosis and prostate cancer.